The present invention is related to reflective-transmission type thin film transistor liquid crystal display and more specifically related to liquid crystal display wherein the reflective pixel electrode and transmissive pixel electrode composing the whole pixel electrode can be stably connected with the source electrode of thin film transistor which controls a pixel.
Liquid crystal displays can be classified to many numerous types according to the methods of manufacturing and the structures. For example, top gate type and bottom gate type can be deferentiated. For the manufacturing of top gate type LCD, active layer made with semiconductor material is formed before gate insulating layer and gate pattern are formed. But, in case of bottom gate type LCD, the gate pattern and the gate insulating layer is formed prior to the active layer.
On the other hand, the classification of reflective type and transmissive type can be possible. In the transmissive type LCD, the pixel electrode is made with reflective material like metals which reflect light from external environment. But, in the reflective type LCD, the pixel electrode is made with transparent conductive materials containing indium metal oxide lineage like ITO (indium tin oxide). In the transmissive type LCD, independent light source is placed at the backside of LCD panel and thus called backlight, and various colorful images can be presented by controlling the state of liquid crystal of numerous pixels to transmit or to shied the light from the backlight.
Recently, reflective type LCD is considered as an alternative in notebook LCD which need large display size and high quality image. And reflective-transmission type LCD which has merits of both reflective type and transmission type is also presented by SHARP(registered trade mark).
In the reflective transmission type LCD of SHARP, transmissive pixel electrode is formed to be connected to source electrode of thin film transistor in the process of making thin film transistor on glass substrate. And passivasion layer which has contact hole in the source region of the thin film transistor is formed on the thin film transistor. Then, reflective metal layer like aluminum is formed on the passivation layer and patterned to confine the reflective area and to form a window which reveals some part of transmissive layer. Here, the patterned reflective metal, the reflective pixel electrode is connected to the source electrode through the contact hole.
Meanwhile, in the process of fabricating the reflective transmission type LCD, some problems can be caused according to the materials adopted as transmissive pixel electrode and reflective pixel electrode. For example, if the reflective pixel electrode made with aluminum, is electrically connected with the source electrode by the transmissive pixel electrode which covers the source electrode. But, the transmissive pixel electrode is made with ITO (indium tin oxide) generally used as transparent conduction layer, an oxide insulation layer is to be formed between the reflective pixel electrode and transmissive pixel electrode. Then, the oxide layer prevents the enhancement of electric field to the reflective pixel electrode.
To avoid the problem, a parameter conduction layer can be used at the interface of the two problematic electrode materials. Also, the two problematic pixel electrode may be connected to the source electrode at different positions to prevent the problem. However, compared with the relatively large pixel electrode area, the region of the source electrode is confined to relatively small area, which makes it difficult to design and to form the transmissive area and the reflective area properly and simply.
It is therefore an object of the present invention to provide a reflective transmission type TFT LCD wherein the stable operation of whole pixel electrode and safe enhancement of electric potential to whole pixel electrode can be confirmed.
It is another object of the present invention to provide a reflective transmission type TFT LCD wherein the easiness and simplicity can be sustained in the process of is forming the whole pixel electrode.
In order to obtain the above mentioned and other objects, the present invention provide a reflective transmission type TFT LCD wherein each of the reflective pixel electrode and the transmissive pixel electrode is connected directly to a source electrode of a thin film transistor, or the transmissive pixel electrode are concurrently formed with gate electrode and made with double layer of transparent conduction layer and metal layer which can be used as parameter conduction layer between the transparent conduction layer and the reflective pixel electrode.
According to one aspect of the present invention, the reflective transmission type thin film transistor liquid crystal display (TFT LCD) comprises a glass substrate, at least one thin film transistor on the substrate for controlling a pixel, passivation layer having at least one contact hole in a source region of the thin film transistor, a transmissive pixel electrode which is formed on the passivation layer and is connected with a source electrode of the source region through a contact hole, a reflective pixel electrode which is formed on the passivation layer and is connected with the source electrode of the source region through a contact hole. And the pixel area is composed of a transparent area in which only the transmissive pixel electrode of whole pixel electrode exist and a reflective area in which the reflective pixel electrode exist.
Here, it is obvious to those who are skilled in the art that the structure of thin film transistor can be varied according to the method of fabrication.
Here, for the prevention of defective chemical reaction among the reflective layer, the transmissive layer and the etchant in the process of etching or cleaning, the reflective transmission type thin film transistor liquid crystal display may have a separating insulator layer which have good transparency between the transmissive pixel electrode and the reflective pixel electrode.
If the number of contact hole is one, the transmissive pixel electrode (in case of comprising the separating insulator, the separating insulator also) have a hole revealing some part of the source region and the reflective pixel electrode which is formed over the transmissive pixel electrode is connected with the source electrode of the part of the source region via the hole.
In case that the number of contact hole is two, one is for the transmissive pixel electrode and the other is for the transmissive pixel electrode, in the region of the other contact hole, the transmissive pixel electrode (in case of having a separating insulator, the separating insulator also) has a hole revealing the source electrode for a connection between the reflective pixel electrode and the source electrode.
Here, the hole formed in the transparent conduction layer and in the separating insulator can be formed by sequential forming of the two layers and patterning of the two layers using one photo mask Generally, the width of the contact hole formed in the source region is far wider than the thickness of layers forming the pixel electrode, the removal of some part of the transmissive conductor layer and the separating insulatior at the contact hole can be executed in the patterning.
In the present invention, the reflective pixel electrode may be made with metals including aluminum and the transmissive pixel electrode may be made with indium metal oxide lineage such as ITO, IZO.
Generally, the transparent area takes form and shape of window in the reflective area. And, the separating insulator can be removed for the efficiency of transmission of backlight by etching process wherein the reflective pixel electrode acts as an etching mask.
In the above explanation, for the efficiency of reflection, the transmissive pixel electrode are preferably formed before the reflective pixel electrode are formed. But, the sequence of forming the transmissive pixel electrode and forming the reflective pixel electrode can be changed with no significant change of effect.
According to another aspect of the present invention, the reflective transmission type thin film transistor liquid crystal display (TFT LCD) comprises a glass substrate, at least one thin film transistor on the substrate for controlling a pixel, a first type pixel electrode which is formed and patterned concurrently with gate of the thin film transistor, comprising upper non-oxidizing metal layer removed in transparent area and lower transparent conductor layer. It also comprises a passivation layer which is formed over the thin film transistor and over a first type pixel electrode. The passivation layer has a contact hole at a source region and a hole (or a window) in the transparent area and neighboring region (in a region including the transparent area). A second type pixel electrode which is formed over the passivation layer, is patterned to cover whole pixel area except the transparent window region, and is connected with a source electrode of the thin film transistor through the contact hole and connected with the non-oxidizing metal layer of the first type pixel electrode at the neighboring region.
Here, the passivation layer is preferably made with photo-sensitive transparent insulator and at least at some part the surface of the passivation layer is treated to take embossment which forms micro lens for focusing or integrating light. More preferably, the thickness of the passivation layer times the difference of refraction index corresponds to a quarter of wavelength of light in liquid crystal.
And the non-oxidizing metal can be one selected form the group consisting of chromium and tungsten-molybdenum.